In 1755 LeRoy passed the discharge of a Leyden jar through the orbit of a man who was blind from cataract and the patient saw "flames passing rapidly downwards." Ever since, there has been a fascination with electrically elicited visual perception. The general concepts of electrical stimulation of retinal cells to produce these flashes of light or phosphenes has been known for quite some time. Based on these general principles, some early attempts at devising a prosthesis for aiding the visually impaired have included attaching electrodes to the head or eyelids of patients. While some of these early attempts met with some limited success, these early prosthesis devices were large, bulky and could not produce adequate simulated vision to truly aid the visually impaired.
As intraocular surgical techniques advanced, however, it became possible to apply a more focused stimulation on small groups and even on individual retinal cells to generate focused phosphenes through devices implanted within the eye itself. This has sparked renewed interest in developing methods and apparatuses to aid the visually impaired. Specifically, great effort has been expended in the area of intraocular retinal prosthesis devices in an effort to restore vision in cases where blindness is caused by photoreceptor degenerative retinal diseases such as retinitis pigmentosa and age related macular degeneration which affect millions of people worldwide.
One such device is described in U.S. Pat. No. 4,628,933, issued to Michelson on Dec. 16, 1986, for a METHOD AND APPARATUS FOR VISUAL PROSTHESIS. The Michelson '933 apparatus includes an array of photosensitive devices on its surface which are connected to a plurality of electrodes positioned on the opposite surface of the device to stimulate the retina. These electrodes are disposed to form an array similar to a "bed of nails" having conductors which impinge directly on the retina to stimulate the retinal cells. The Michelson '933 device is powered by a separate circuit through electromagnetic or radio frequency induction. To receive this energy, an inductor is included with the Michelson '933 device either wound on the periphery of the device or formed on one of the surfaces through photolithographic circuit techniques. The induced signal is then rectified and filtered to power the circuit elements.
Such a device, however, increases the possibility of retinal trauma by the use of its "bed of nails" type electrodes which impinge directly on the retinal tissue. Additionally, by including the photosensitive elements integral with the implanted device within the eye, the processing of the perceived image is quite limited. It is first limited by the quality of the lens of the patient's eye and her ability to focus that lens. If the lens is occluded by cataracts or otherwise not clear, it will need to be replaced to allow proper operation of the prosthesis. Likewise, prescription glasses or contact lenses may be required to focus the image on the prosthesis for patients who would otherwise be near or far sighted.
The Michelson '933 device is also limited by the physical size available within the ocular cavity. Since this cavity is small and since the device must be supported by the retinal tissue itself, the amount of image processing circuitry which can be included therein is limited. Furthermore, the amount of image processing circuitry is also limited by the power availability and utilization requirements within the ocular cavity. As a result of these limiting factors, the Michelson '933 device does not include any image processing circuitry other than common signal amplifiers which simply tune the responses to the frequency response bandwidth of the retinal neurons, to shape the output waveform in a charge-balanced square wave, and trim the voltage and current output to acceptable levels for the neurons.